1. Field of the Disclosure
Embodiments of the disclosure relate to a DC-DC converter for outputting a desired voltage at a constant level.
2. Discussion of the Related Art
In general, a switching regulator is a device for converting energy using an energy storage device, such as an inductor and a capacitor. A DC-DC converter suitably feeds back to the switching regulator and steps up or down an input DC voltage, thereby outputting a DC voltage of a desired level. Examples of the DC-DC converter include a buck converter and a boost converter. The buck converter is a step-down converter to convert a high DC voltage into a relatively low DC voltage, and the boost converter is a step-up converter to convert a low DC voltage into a relatively high DC voltage.
The DC-DC converter has to suitably select circuit elements, such as the inductor and the capacitor, and has to be controlled by a suitable manner. A pulse frequency modulation (PFM) manner and a pulse width modulation (PWM) manner are known as a basic control manner used in the DC-DC converter. The PWM manner has advantages of a small noise and a small ripple and has been used in most of the DC-DC converters.
FIG. 1 illustrates an example of a related art DC-DC converter controlled by a PWM manner. More specifically, FIG. 1 schematically illustrates a step-down converter. As shown in FIG. 1, the related art DC-DC converter includes a switching unit 1, a PID controller 2, a comparator 3, and a switching controller 4. The switching unit 1 includes a charge switch and a discharge switch which are reversely turned on and off in response to a switching control signal received from the switching controller 4, an inductor for storing charged energy, a capacitor for storing discharged energy, etc. and produces an output voltage Vo. The PID controller 2 includes an error amplifier. The PID controller 2 compares the output voltage Vo input to the error amplifier with a previously determined reference voltage and produces a PID control voltage Va. The comparator 3 compares the PID control voltage Va with a ramp voltage Vramp and produces a switching control voltage Vc. The switching controller 4 turns on the charge switch and at the same time turns off the discharge switch in an on-period of the switching control voltage Vc, which is intermittently produced. The switching controller 4 turns off the charge switch and at the same time turns on the discharge switch in an off-period of the switching control voltage Vc.
An operation of the related art DC-DC converter is described in detail. As shown in FIG. 2, when the charge switch is turned on and the discharge switch is turned off in the on-period of the switching control voltage Vc, an input voltage Vi applied by a battery source is charged to the inductor at a slope of (Vi−Vo)/L1, where L1 is an inductance of the inductor. Afterward, when the charge switch is turned off and the discharge switch is turned on in the off-period of the switching control voltage Vc, energy charged to the inductor is discharged to a load and the capacitor at a slope of (−Vo/L1). The related art DC-DC converter periodically repeats the charging operation and the discharging operation, thereby holding the output voltage Vo constant. The output voltage Vo is determined by the input voltage Vi and a ratio (i.e., a duty ratio) of on-time to off-time of the charge switch. Thus, a final output of the related art DC-DC converter is expressed by an Equation of Vo=D*Vi, where 0<D<1. FIG. 2 illustrates an on-rate and an off-rate of the charge switch depending on the ramp voltage Vramp and the PID control voltage Va. In FIG. 2, when the PID control voltage Va increases because the output voltage Vo is lower than the reference voltage, a width of the on-period of the switching control voltage Vc increases. On the contrary, when the PID control voltage Va decreases because the output voltage Vo is higher than the reference voltage, a width of the off-period of the switching control voltage Vc increases. The output voltage Vo is controlled through the above-described process, so that it is held at a desired constant level.
The related art DC-DC converter holds the output voltage Vo constant through a negative feedback configuration. However, the related art DC-DC converter does not generate the output voltage Vo depending on operational conditions in an initial drive. More specifically, when the related art DC-DC converter operates for the initial drive, the comparator 3 compares the PID control voltage Va with the ramp voltage Vramp and generates the switching control voltage Vc. The switching control voltage Vc is applied to the switching controller 4 and is used to control switching operations of the charge switch and the discharge switch. However, in the initial drive, the PID control voltage Va output through a node Na has different initial values depending on the operational conditions of the initial drive because of negative feedback characteristics through the PID controller 2. As a result, the related art DC-DC converter shows a conditional situation where the output voltage Vo is not generated depending on the operational conditions because of an influence of the different initial values of the PID control voltage Va.